Despite the advent of antiviral chemotherapy, genital herpes simplex virus (HSV) infection continues to be epidemic throughout the world. Much of this relates to the lack of understanding of the natural history of the subclinical carrier of HSV. Our recent studies indicate that HSV reactivation as detected by PCR is 10 times more frequent than previously appreciated and that subclinical shedding accounts for the majority of transmission to sexual partners. The specific aims are 1) to define the frequency and anatomic sites of subclinical HSV reactivation and to define the virologic and immunologic correlations of subclinical clusters of HSV reactivation; 2) to determine the rate, the sites, and the determinants of subclinical HSV-2 reactivation in men; 3) to define the frequency of symptomatic and subclinical reactivation of genital HSV-1 infection and to compare the virologic and host immune response to genital HSV-1 infection with oral HSV-1 infection and with genital HSV-1 infection; to estimate the frequency of HSV-1 transmission associated with oral-genital contact among serologically discordant lesbian couples; 4) to identify and prospectively follow cohorts of men and women at risk for HSV-2 infection to estimate the incidence of HSV-2 infection and to define the virologic, host and behavioral risk factors associated with HSV-2 acquisition; the cohorts will comprise patients at an STD Clinic, high-risk adolescents, gay men and stable partners of HSV-2 seropositive persons. The research design includes prospective, detailed, virologic and immunologic study of HSV-1 and HSV-2 infected persons using samples collected daily from multiple anatomic sites for quantitative viral cultures and PCR. Prospective epidemiologic studies of HSV transmission and acquisition will use repeat serologic testing, clinical examinations and behavioral diaries. A subset of persons at risk for transmitting HSV will be followed with daily viral cultures to assess the risk of transmission from symptomatic and subclinical HSV reactivation. Information from this project will be of direct benefit to the design and conduct of candidate genital HSV vaccines.